You know how you spill coffee, you don’t just get a uniform stain? Instead, you get a stain with a dark ring around the edge. In the image to the left, you can see this in the coffee “blotches” (the round rings are caused by the coffee that was stuck to the bottom of the round mug).

It turns out that, as of 10 years ago, this was a question that wasn’t actually well-understood scientifically. So Sid Nagel set out to figure it out. Sid Nagel’s a condensed matter physicist who is famous for setting his mind to variety of interesting fluid problems. In particular, he studies the interesting properties of granular materials (like, why does sand sometimes flow like a fluid and sometimes jam-up like a solid?). I’ll have to write a post just on that sometime in the future!

Anyway, back to coffee rings. The puddle of spilled coffee starts to evaporate. But something interesting happens as it evaporates, Nagel found. If you could look at the coffee particles under a microscope as it dries, you’d see the particles flowing from the center out to the edge. Why’s that? It turns out that the edge can’t “retreat” as it dries… the edge of the drop is stuck where it first falls, for the most part. Here’s why. Say you spilled the coffee on some paper. The paper is a bit rough. The edge of the drop gets “pinned” in place by that roughness. So that means that the drop gets flatter as it dries (rather than shrinking in width).

So as the edge loses liquid by evaporation, liquid from the center of the drop has to flow outward to replenish it. So the coffee particles constantly flow out towards the edge of the drop, the water dries out, and more coffee particles flow to the edge. So when the entire drop is dried out, you have a dark ring around the edge! This isn’t particular to coffee — any dark liquid will do.

Nagel started this research by noticing the interesting behavior of the coffee stains on his countertops!

One neat way to show that this explanation works is to try to keep the drop from evaporating at the edge. You can do this by covering the whole drop except for a little hole at the center. If you do this… no ring! Try it.

Here’s an article about it. And some neat Q&A with Sid Nagel about this and other condensed matter stuff (like the Brazil nut effect), including how to make the Coffee Stain Font!

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No, the myth isn’t that airplane fly at all — we know they do, but how do they do it? This is one that really bothered a bunch of us when we were in graduate school in physics. How does an airplane fly? I have a substantial investment in knowing that the physics of these big metal monsters is sound. The reason we were worried is the Bernouilli effect. The Bernouilli effect is what sucks papers out of your car window when you’re speeding down the highway — it says that the faster a fluid (e.g., air) moves, the lower its pressure. That’s why the papers get sucked out of the window — they’re drawn towards the lower pressure outside the window, where the air is moving quickly.

For airplanes, the Bernouilli argument goes that the air moving over the top of the wing (where it’s curved, see below) must travel farthe than that moving under the wind (where it’s flat). So, the lift is caused by the lower pressure on the top of the wing relative to the bottom of the wing.

Fine. But then how do planes fly upside down?

The Bernouilli argument above is flawed. There is no reason why two air molecules which hit the front of the wing at the same time must rejoin each other at the trailing edge, which is what the above argument suggests with its “air must go faster along the top of the wing because it’s traveling further than if it had gone below the wing.”

The key lies instead in the “angle of attack” shown in the above diagram. The wing is slanted upward slightly. As the wing moves forward, it pushes air in front of it, which “piles up” under the wing, becomes compressed, producing high pressure on the underside of the wing. At the same time, the upper surface is being pulled away from the air behind it as the plane moves forward. This leaves a low pressure area along the upper surface of the wing. This produces lift.

Another force lifting the wing is that the lower surface of the wing hits air molecules downward as it moves. Every action produces an equal and opposite reaction, so just as when two balls hit each other and move off in opposite directions, the wing hits air downward and this throws the wing upward slightly. This gives some more lift.

This post was adapted from Kenneth Fuller’s website.

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I’ve got a new podcast episode… In this episode one of the teachers in our teacher workshops showed us one of his favorite activities, using the sound of paperclips (attached at intervals on a string) to estimate the rate of free fall. It’s a really elegant little experiment!

Click this link to check it out:
36. Stringing Us Along

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Gosh, I’d like to believe this one, it’s just such a “cool” idea. One argument against this idea is that if you take thousands of pictures of snowflakes, it’s still not a very good statistical sample. Kenneth Fuller writes about this, and other modern myths taught as science. He hypothesized that this myth arose from the publication of a wide sample of snow crystals by Wilson Bently in 1931. Bently only published his very best pictures, which were taking from a specific type of storm. While the final result was astounding (6000 photographs), this is not a very good sample when you consider all the snowflakes that have ever existed in the world.

A counter-argument (which Fuller rejects) is that on the molecular level, no two snowflakes will ever be quite the same, because some of the water molecules will be slightly different from the others (e.g., they’ll contain an isotope of hydrogen or oxygen). Fuller poo-poos this idea, since it also says that no two drops of water are exactly alike, which begs the question of whether or not the beautiful symmetrical crystal, above, might be replicated from one snowflake to the next.

However, the math of combinatorics comes into play when you consider all the different ways that a snow crystal might form. By that, I mean that as each snow crystal forms, it has several different “choices” about how to continue its growth. There are many crystal structures available, and different paths that each crystal may take as it continues its growth. So, there is a huge number of different crystal structures that could arise. Snow researcher Kenneth Librecht from Caltech claims at Snowcrystals.com that it’s statistically unlikely that two snowflakes might be exactly the same (even though we could never actually check them all to make sure).

See some beautiful (public domain) pictures of snowflakes at Wikimedia Commons.

Find out how to preserve a snowflake for 30 years with superglue on a later post.

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Hi everyone, I posted an episode to my podcast, Science Teaching Tips.

Click this link to check it out:
35. When Words Fail You

- Stephanie

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The answer: yes and no. When applied to toilets and sinks, this is one of those “too good to be true” science factoids, I’m afraid. But it does apply in some situations.

The myth goes that if you flush a toilet in Australia the water swirls down the drain the opposite way than in the northern hemisphere, due the Coriolis effect (an apparent force which describes how objects veer to the left or right when traveling on something that’s rotating — see the link above for a good visualization of this).

If there were no other forces on that water in the sink or toilet, that would be true. The Coriolis effect does actually make hurricanes rotate the opposite direction in the two hemispheres. But for toilets and sinks it’s another story. The toilet myth is easy to dispell — just peek around the rim of the toilet and you’ll see that the water is jetted into the bowl at an angle, which determines the direction the water swirls. Sinks, however, are a little more tricky.

I’ve heard of charlatans who hang around the equator in Kenya, carrying basins of water. They’ll stand on the southern side of the equator with the basin, pull a plug at the bottom, and show that it swirls out counter-clockwise. Then they’ll walk to the northern side of the equator, fill the basin and pull the plug, and it swirls out clockwise. Irrefutable proof? Be careful! You have to know all the initial conditions in any experiment, and in this one, there is one that is hidden from you. The huckster just has to add a slight rotation to the water before letting it out (for example, pour the water in at a very slight angle to give it an initial rotation, and it will “remember” that rotation as it swirls out of the basin. In fact, you can swirl the water in the basin, then walk away from it for several hours, and it will still “remember” that rotation when you pull the plug! Plus, the charlatans got it backwards — water should actually swirl counter-clockwise in the northern hemisphere if the Coriolis effect were at play! (See Alistair Fraser’s website for a great explanation of how you can re-create this fakery for a fun party trick!)

Even if you don’t give the water an initial swirl, tiny pits and imperfections in the basin can give the water a rotation — which may be clockwise or counterclockwise, but doesn’t depend on which hemisphere you’re in!

Now, all that said, the Coriolis effect does play a role. It’s just really tiny relative to all these other effects. It’s about 10 millions the size of the effect of gravity. So, if you have a perfect basin, with completely still water, then the Coriolis force will make the water swirl opposite directions in the two hemispheres. This was demonstrated by Ascher Shapiro, a researcher at MIT in 1962. You can see the Straight Dope talk about this topic, or detailed information from Alistair Fraser.

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I’ve posted a new episode to my podcast, Science Teaching Tips. I’ve always been fascinated with sound — there are a lot of neat things you can do with sound, and some little known facts. This podcast is a bit of a smattering of some of the fun things I found out about sound while I was at the Exploratorium.

Click this link to check it out:
34. Sound Bytes (Part 2)

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